1. Field of Invention
The present invention relates to oil and gas production. More specifically, the present invention relates to a tool that creates a shockwave in a wellbore to “back-off” threads engaged in a threaded couplings within a tubular string.
2. Description of Prior Art
Typically, tubulars are connected together by threaded couplings to form a string that is suspended and cemented in a wellbore to create a casing for the wellbore. From time to time, the casing string may need to be removed from the wellbore and the threaded couplings are decoupled at surface. In some instances while removing the casing it may become wedged within the wellbore; further complicating string removal, while still downhole, one of the threaded couplings may resist detachment under an applied torque to become immovable. The immovable coupling is sometimes unseated by directing a shockwave at the coupling site to break loose the threaded connection.
A typical prior art tool used to create this shockwave consists of multiple strands of detonator cord wrapped around a shot rod in a rope-like fashion and wrapped with friction tape. Generally this tool employs a detonation cord having HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine), which can withstand operating temperatures of 400 degrees Fahrenheit for only about an hour. While a detonating cord having HNS (1,3,5-Trinitro-2-[2-(2,4,6-trinitrophenyl)ethenyl]benzene) can operate at temperatures above those limiting use of HMX detonator cord, HNS detonating cord cannot side detonate and thus is not utilized in the above described prior art tool. Also, operating pressure of typical prior art is limited to 20,000 psi due to the use of exposed (to wellbore fluids) interface between detonator and detonating cord.